In telecommunications networks, reliability and failure resistance is a primary issue and telecommunications equipment shall ensure continued operation even in the case any equipment component fails. Therefore, network elements are provided with equipment protection, which means that core components are provided twice so that a redundant system component can take over operation of a failed system component.
Typical network elements of a transmission network such as crossconnects and add/drop multiplexers have a switching matrix for randomly connecting signals from any input to any output. This switching matrix (referred to as copy A) is typically protected by a redundant second matrix (referred to as copy B).
The principle of equipment protection is that all signals are duplicated and distributed to the active and the redundant standby equipment component, which both perform the identical operation on the signal. On the output side, the two signals are combined again by selecting the better of the two for further processing. So, if the active component fails, the signal processed by the standby component is selected and the standby component thus becomes active.
However, loss of signal detection may take some time, during which a bit error burst is transmitted before the protection switch is activated. This situation results in a “hit” in the output signal and it is thus desirable to perform hitless protection switching, i.e., to switch from active to standby signal before a disruption in the active signal may reach the output.
Known hitless protection switch systems have a large buffer for synchronization of the data signals. This buffer allows to base the loss of signal detection on a loss of frame event. Known failure detectors therefore need a long time (expressed in bit periods) to detect a failure condition and switch from active to standby signal. Moreover, a large buffer impacts the signal delay performance during normal operation and involves higher equipment cost and higher power dissipation.
It is thus an object of the present invention to provide a hitless equipment protection system and method that does not require large buffers.
European patent application EP 0 696 111 A2 describes a hitless path switching apparatus where the working and protection paths are continuously monitored for bit errors. If a bit error occurs in the working path and no bit error occurs in the protection path, switching from the working to the protection path is performed on data block basis.
European patent application EP 1 261 157 A2 describes hitless protection switching in a transmission system supporting virtual concatenation. The described apparatus receives data signals on at least two transmission paths and outputs data from a selected one of these paths. The received data signals are aligned first so that the selector mechanism can select between corresponding elements of the received data signals. This allows hitless switching to be performed since there is no loss or repetition of signal elements when switching occurs.
German patent application DE 42 38 410 A1 describes a communication and control system in a base station of a cellular network. All modules of the base station communicate over a common bus system. Two redundant bus systems are provided and all data communicated in parallel over both of them. Check bits are evaluated to select only valid data from the two busses.
German patent application DE 39 29 793 A1 describes duplex communication over redundant communication paths. In receive direction, the paths are monitored for the presence of a carrier signal. When no modulated carrier signal is received on the selected path, a modulated carrier is sent in reverse direction step by step on all paths. When a carrier signal is detected on one of the paths, this one is selected as active path.